Spread spectrum modulated signals (CDMA, Code Division Multiple Access) are used, for example, in global navigation satellite systems (GNSS), such as the GPS system, as well as in many third generation mobile communication systems, such as the UMTS. For generating a spread spectrum modulated signal, the modulation is performed in a transmitter by using an individual spreading code, wherein several transmitters can simultaneously transmit a signal at the same frequency, when each transmitter is allocated a unique spreading code. For example, in satellite positioning systems, each satellite uses a spreading code of its own. In the receiver, the corresponding reference code is generated or it is read from the memory of the receiver, and this reference code is used for searching the received signal for the signal of the transmitter which is to be received. For successful signal reception, the receiver must perform acquisition of the signal, typically by using several correlators and controlling the code phase and frequency of the reference code, wherein the signals generated by the correlators are used to determine the correct code phase and the frequency shift. After the acquisition has been completed, the tracking of the signal is continued so that the reception of the signal and the demodulation of the information transmitted therein would be possible. In this tracking step, the code phase and frequency of the reference code are to be kept locked with the code phase and frequency of the signal to be received.
The acquisition and the tracking of the signal are problematic particularly indoors where the strength of the signal to be received is poor, possibly even lower than background noise. Such a situation occurs particularly in satellite positioning systems, in which the signal to be received is very weak upon arrival on the earth, and indoors this signal can be further attenuated by the walls of buildings. To cure this problem, solutions of prior art are aimed at implementing the receiver by providing it with a large number of correlators and by using a long integration time. At present, receivers may comprise as many as about 16,000 correlators. For the sake of comparison, it should be mentioned that the first portable GPS receivers only comprised 12 or even fewer correlators. The increase in the number of correlators naturally also means that the circuit board area required for implementing the correlators is significantly increased as well. Furthermore, this increases the power consumption of the receiver. Because of the higher power consumption, the heating of the device may also be increased.